RMS Power

[Bassulike66]

Can someone enlighten me on RMS??

Example; If i read this on the back of an amp, Power Handling Capacity: 700 Watts RMS.

Would i be mistaken if i understand it be a load of poo, and misleading!??!

Without trawling the internet for long drawn out conspiracy theories...was RMS first used by the Americans detailing the spec on "HI-FI systems" back in the late 70's? and somehow got it wrong

..thus misleading the world into thinking they have massive sound handling capacity in their amplifiers??

[Stewart]

Negligible trawling reveals:

[url="http://en.wikipedia.org/wiki/Audio_power"]Audio Power definitions (including RMS)[/url]

[obbm]

You've got it the wrong way round.

RMS power is the correct measurement and terminology.

The term Peak Music Power or Peak Program Power is used to make you think that you have twice the amount of power you actually have.

[Mr. Foxen]

There is a 'but' in that the rating is for the amount of power you can be putting into an amp before the voice coil overheats. If could be farting and tearing at a lower power, depending on its excusrion limited power handling, which is specific to frequency, so not as handy to quote.

[obbm]

[quote name='Mr. Foxen' post='786030' date='Mar 25 2010, 04:51 PM']There is a 'but' in that the rating is for the amount of power you can be putting into an amp before the voice coil overheats. If could be farting and tearing at a lower power, depending on its excusrion limited power handling, which is specific to frequency, so not as handy to quote.[/quote]


Don't you mean "out of an amp into a speaker"? The only power going into an amp is from the AC mains.

[Mr. Foxen]

[quote name='obbm' post='786036' date='Mar 25 2010, 04:59 PM']Don't you mean "out of an amp into a speaker"? The only power going into an amp is from the AC mains.[/quote]

Yes, hurried work typing. Actually, mostly confused by phrasing of question 'power handling on back of amp'. Back of cab = speakers thermal power handling.

[Phil Starr]

Music in your amp is turned into a voltage that goes up and down above and below zero, mainly following a curve called a sine wave. Because the volts go - and+ equally the average is zero over a full cycle. Squaring the voltage makes everything positive then you can find the average voltage and turn this back into volts by finding the square root (hence Root Mean Square). It is just a mathematical trick to deal with an alternating voltage or current so you can compare it with direct current or voltage. You don't need to understand this but it might help.

Measuring the output of an amp is easy. You just pass the output through a known resistance and measure the voltage drop at the point where the amp clips. There are all sorts of regulations about the detail of doing this to ensure your amp gives something like this in the real world but this is the basics.

Speakers are tricky to rate. Music has loud and quiet bits and all sorts of frequencies in it. Speakers are tested with white noise (all the frequencies) filtered as pink noise which is meant to have the same sort of frequency content as typical music! Speakers are rated continuously and if they were rated at 35W should handle 35W of pink noise for hours at a stretch and as Mr Foxen has said this is about their ability to handle heat and not music. The reality is that the tests are far worse than anything you are ever likely to do to them and most speakers can handle more than their rated power.

The other problem with speakers that is not mentioned so much is that cones can only travel back and forward a limited amount and a few watts of deep bass can make them reach this limit. All bass guitar speakers are designed to stop this from damaging the speaker and th better designers make the limiting forces naturally compress the sound to avoid the farting noise of a speaker reaching it's limits but the excursion limit or Xmax means many bass speakers give far less bass at high power than their specs would show.

Anyone advertising the rms wattage of an amp or speakers is liable in law for the claims they make so these are hte only reliable measurement to use. RMS may be described as EIA in the US as this is the body that designs the test regime.